1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system for an internal combustion engine which is provided with a device capable of changing air intake characteristics of the engine in dependence on operating conditions of the engine.
2. Prior Art
There is conventionally known a method of controlling the air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine in a feedback manner responsive to outputs from first and second exhaust gas component concentration sensors for sensing concentration of an exhaust gas component, arranged in the exhaust passage of the engine at respective locations upstream and downstream of an exhaust gas-purifying catalytic converter arranged in the exhaust passage, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 5-321721.
Further, an air-fuel ratio control system for internal combustion engines, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 6-74081, is also conventionally known, which is provided with first and second catalytic converters serially arranged in the exhaust passage in this order from the upstream side of the exhaust passage, a bypass passage bypassing the first catalytic converter, a changeover valve for changing the flow passage of exhaust gases between one passing through the first catalytic converter and one passing through the bypass passage, and a valve operation-changeover mechanism for changing the operative states of intake valves and exhaust valves of the engine, wherein the fuel supply amount and the basic control amount of ignition timing are changed in dependence on the operative state of the changeover valve as well as the operative states of the intake valves and exhaust valves.
However, according to the control system known from Japanese Laid-Open Patent Publication (Kokai) No. 6-74081, changeover of the operative states of the intake valves and exhaust valves causes a change in air intake characteristics of the engine, resulting in a change in transfer delay of exhaust gases. As a result, if the air-fuel ratio feedback control method known from Japanese Laid-Open Patent Publication (Kokai) No. 5-321721 is directly applied to the above control system, the controllability of the air-fuel ratio can be degraded, resulting in temporary degradation of exhaust emission characteristics of the engine.